Oxyfuel Combustion Boiler Plant and Operation Method of Oxyfuel Combustion Boiler Plant

ABSTRACT

An oxyfuel combustion boiler plant comprising:
         a boiler having an air separation unit for manufacturing oxygen by separating nitrogen from air, a burner for burning the oxygen supplied from the air separation unit and pulverized coal, and a primary system pipe for supplying the pulverized coal to the burner, exhaust gas recirculation system pipe for supplying combustion exhaust gas discharged from the boiler to the primary system pipe, a carbon dioxide capture unit for capturing carbon dioxide in the exhaust gas discharged from the boiler, the oxyfuel combustion boiler plant is further comprising: an oxygen buffer tank disposed on a downstream side of the air separation unit; an oxygen supply pipe for supplying oxygen to the primary system pipe of the burner from the oxygen buffer tank; and a nitrogen supply pipe for supplying a part of nitrogen generated from the air separation unit or an air supply pipe for supplying air from outside which is connected to the oxygen supply pipe on a downstream side of the oxygen buffer tank and on an upstream side of a junction of the primary system pipe.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial No. 2009-225912, filed on Sep. 30, 2009, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxyfuel combustion boiler plant andan operation method of the oxyfuel combustion boiler plant.

2. Description of Related Art

A boiler for burning pulverized coal can be divided into two kinds ofcombustion systems depending on the gas to be supplied to a burner.Air-fuel combustion is of a system for burning fuel by supplying air tothe burner. Further, oxyfuel combustion is of a system for burning fuelby a mixture of high purity oxygen and combustion exhaust gas instead ofair.

In the oxyfuel combustion, the exhaust gas components are mostly carbondioxide, so when capturing carbon dioxide from exhaust gas, there is noneed to concentrate the carbon dioxide. Therefore, the oxyfuelcombustion can pressurize and cool the exhaust gas as it is and liquefyand separate the carbon dioxide, so it is one of the valid methods ofreducing the discharge rate of carbon dioxide.

In the oxyfuel combustion system, as a method for accelerating ignitionof pulverized coal flowing in the vicinity of the burner, a method forinjecting oxygen toward a mixture flow of pulverized coal and combustionexhaust gas is proposed (Patent Document 1).

Patent Document 1: Japanese Patent application Laid-open No. Hei 7(1995)-318016

SUMMARY OF THE INVENTION

However, when injecting oxygen toward the mixture flow of pulverizedcoal and combustion exhaust gas, there is the possibility thatpulverized coal may enter a mass of gas having a high oxygenconcentration and abnormal combustion such as a backfire may occur.Particularly, immediately after starting oxygen supply or when changingthe operation conditions, abnormal combustion occurs easily.

Therefore, an object of the present invention is to provide an oxyfuelcombustion boiler plant or an operation method of the oxyfuel combustionboiler plant that prevents abnormal combustion from occurring in theburner.

The present invention provides an oxyfuel combustion boiler plantcomprising: an oxygen buffer tank disposed on a downstream side of theair separation unit; an oxygen supply pipe for supplying oxygen to theprimary system pipe of the burner from the oxygen buffer tank; and anitrogen supply pipe for supplying a part of nitrogen generated from theair separation unit or an air supply pipe for supplying air from outsidewhich is connected to the oxygen supply pipe on a downstream side of theoxygen buffer tank and on an upstream side of a junction of the primarysystem pipe.

According to the present invention, an oxyfuel combustion boiler plantor an operation method of the oxyfuel combustion boiler plant thatprevents abnormal combustion from occurring in the burner can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing for illustrating the constitution of an oxyfuelcombustion boiler plant of the first embodiment,

FIG. 2 is a drawing for illustrating the relation between the oxygenconcentration in the oxidizing gas and the flame propagation velocity ofpulverized coal under various atmospheric conditions,

FIG. 3 is a drawing for illustrating the situation of changes of thefuel supply rate, air supply rate, oxygen supply rate, and recirculationrate of combustion exhaust gas until the plant starts in the air-fuelcombustion, then switches to the oxyfuel combustion, and reaches thestationary operation of capturing carbon dioxide,

FIG. 4 is a comparison example, when the plant is operated according tothe plan shown in FIG. 3, showing the situation of changes of the oxygenconcentration in the oxidizing gas,

FIG. 5 is a drawing showing an example of the situation of changes ofthe oxygen concentration in the oxidizing gas, when the plant isoperated in the first embodiment,

FIG. 6 is a drawing showing an example of the shifts between the planedsupply rate and the actual supply rate when oxygen is supplied of thefirst embodiment,

FIG. 7 is a drawing showing changes of the maximum values of the localinstantaneous oxygen concentration in the oxidizing gas when the plantis operated according to FIG. 6 in the first embodiment,

FIG. 8 is a drawing showing an example of the shifts between the planedsupply rate and the actual supply rate when oxygen and nitrogen or airare supplied in the first embodiment,

FIG. 9 is a drawing showing changes of the maximum values of the localinstantaneous oxygen concentration in the oxidizing gas when the plantis operated according to FIG. 8 in the first embodiment,

FIG. 10 is a drawing for illustrating the constitution of an oxyfuelcombustion boiler plant of the second embodiment,

FIG. 11 is a drawing showing the mixing state of gas and pulverized coalin the second embodiment,

FIG. 12 is a drawing showing the mixing state of gas and pulverized coalwhen the oxygen gas is supplied like stepwise in the second embodiment,

FIG. 13 is a drawing for illustrating the constitution of the vicinityof the burner relating to the third embodiment,

FIG. 14 is a drawing for illustrating a modification of the constitutionof the vicinity of the burner relating to the fourth embodiment,

FIG. 15 is a drawing for illustrating another modification of theconstitution of the vicinity of the burner relating to the fourthembodiment,

FIG. 16 is a drawing for illustrating still another modification of theconstitution of the vicinity of the burner relating to the fourthembodiment, and

FIG. 17 is a drawing for illustrating in the direction A of theconstitution of the burner shown in the FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiments of oxyfuel combustion boiler plantwill be explained with reference to the accompanying drawings.

Embodiment 1

FIG. 1 shows an oxyfuel combustion boiler plant using coal as fuel. Thisembodiment is a thermal power plant for generating steam using a boiler200.

The boiler 200 includes a burner 210 and a gas port 225. The burner 210supplies and burns pulverized coal to the furnace in the boiler. The gasport 225 is installed on the downstream side of the burner 210 andsupplies second stage combustion gas to the furnace.

The system pipes through which combustion exhaust gas discharged fromthe boiler 200 flows will be explained below. Combustion exhaust gas 380indicates the system pipe through which the exhaust gas discharged fromthe boiler 200 flows. An air quality control unit 340 is an apparatusfor purifying exhaust gas. A fan 381 is a unit for letting exhaust gasflow. A dryer 341 cools exhaust gas and simultaneously removeshygroscopic moisture. A CO₂ capture unit 350 compresses exhaust gasafter drying and captures carbon dioxide from the exhaust gas.Uncaptured gas 351 indicates a system line showing gas remaining aftercarbon dioxide is captured by the CO₂ capture unit 350. Circulationexhaust gas 390 indicates a system pipe through which a part of thecombustion exhaust gas 380 discharged from the boiler is re-circulatedto the boiler 200. A circulation exhaust gas flow rate regulating valve391 has a function for adjusting the flow rate of circulation exhaustgas. A fan 382 is a unit for pressurizing the circulation exhaust gas soas to re-circulate it to the boiler. A gas preheater 330 permits thecombustion exhaust gas 380 and circulation exhaust gas 390 to exchangeheat, thereby heating the circulation exhaust gas 390. Low-temperaturecirculation exhaust gas 393 indicates a system line for bypassing thegas preheater 330. A bypass flow rate control valve 394 has a functionfor adjusting the flow rate of the circulation exhaust gas 393. Flowrate regulating valves 213 and 214 have functions for adjusting the flowrates of the circulation exhaust gas 390 at which it is suppliedrespectively to the burner 210 and air port 225. Further, an air pipe363 b for air-fuel combustion supplies external air to the boiler 200 atthe time of air-fuel combustion. On the air pipe 363 b for air-fuelcombustion, a stop valve 135 c and an air flow rate regulating valve 364are installed.

Next, the oxygen supply system will be explained. An air separation unit360 is an apparatus for separating nitrogen from air 363 a andmanufacturing oxygen. Liquefied gas 131 is manufactured by the airseparation unit 360. An oxygen buffer tank 132 is a tank for storing theliquefied gas 131. A carburetor 133 vaporizes the liquefied gas 131 andgenerates oxygen gas 362. A first oxygen supply pipe 219 is a pipe forletting the oxygen gas 362 pass through. On the first oxygen supply pipe219, a flow rate regulating valve 218 for adjusting the flow rate ofoxygen supplied to a primary system pipe 216 is provided. Further, aflow rate regulating valve 211 is a valve for adjusting the flow rate ofoxygen supplied to the circulation exhaust gas. And, a second oxygensupply pipe 397 is a pipe for supplying oxygen to the circulationexhaust gas on the upstream side of a coal mill 130. Also on the secondoxygen supply pipe 397, a flow rate regulating valve 396 is installed.

Next, the nitrogen supply system will be explained. Nitrogen gas 361 isdischarged to a chimney 370. A nitrogen supply pipe 137 is a pipe forsupplying a part of nitrogen gas separated by the air separation unit360 to the first oxygen supply pipe 219. On the nitrogen supply pipe137, a flow rate regulating valve 134 and a stop valve 135 a areinstalled.

Further, to the first oxygen supply pipe 219, an air supply pipe 138 isconnected. The air supply pipe 138 includes a gas supply unit 136 forpressurizing and supplying air 363, a stop valve 135 b, and the flowrate regulating valve 134.

Also, the coal mill 130 pulverizes coal and generates pulverized coal. Aprimary system pipe 216 of the burner 210 supplies the pulverized coalfrom the coal mill 130 to the burner 210 together with circulationexhaust gas. The system for supplying the circulation exhaust gas 390 tothe coal mill 130 includes a flow rate regulating valve 215. Further,the pipe for supplying the circulation exhaust gas 390 directly to theburner 210 is assumed as a secondary system pipe 217.

The system of the combustion exhaust gas 380 discharged from the boilerincludes the gas preheater 330, the air quality control unit forpurifying exhaust gas 340, the fan 381 for letting gas flow, the carbondioxide capture unit 350 for cooling, liquefying, and capturing carbondioxide in the exhaust gas, and the chimney 370 for discharging theuncaptured gas 351 composed of mainly nitrogen and oxygen remainingafter capture of carbon dioxide.

As shown in FIG. 1, the boiler plant of this embodiment includes the airseparation unit 360 for dividing air into gas mainly composed ofnitrogen and gas mainly composed of oxygen and manufacturing high purityoxygen. The air separation unit 360 is a system for separating oxygenand nitrogen using a difference in the boiling points between them andcools air, thereby manufacturing oxygen. This embodiment does not dependupon the air separation method but other methods such as a filmseparation system for separating air using a difference in the sizebetween nitrogen molecules and oxygen molecules.

The air separation unit 360 separates the air 363 a to the high purityliquefied oxygen 131 and nitrogen gas 361 mainly composed of nitrogen.The separated nitrogen gas 361 is discharged into the air from thechimney 370.

If fuel is burnt using high purity oxygen instead of air, the flametemperature becomes excessively high, thus there is the possibility thatthe burner for burning fuel and the wall surface of the boiler may bedamaged. Therefore, high purity oxygen gas manufactured by the airseparation unit 360 is mixed with the circulation exhaust gas 390 whichis a part of exhaust gas discharged from the boiler and is supplied tothe burner 210 and two-stage combustion gas port 225. The circulationexhaust gas 390 is raised in temperature by the gas preheater 330. Apart of the circulation exhaust gas 390 is permitted to bypass withoutpassing through the gas preheater 330 and the low-temperaturecirculation exhaust gas 393 is mixed with the circulation exhaust gas,thus adjusting the temperature. The flow rate of the low-temperaturecirculation exhaust gas 393 is adjusted by the bypass flow rate controlvalve 394.

The circulation exhaust gas 390 is a part of the gas after beingpurified by the air quality control unit 340 and is raised intemperature by the gas preheater 330. The flow rate of the circulationexhaust gas 390 can be adjusted by the opening of the circulationexhaust gas flow rate regulating valve 391.

The flow rate of the oxygen gas 362 supplied to the burner 210 and gasport 225 can be adjusted by adjusting the openings of the flow rateregulating valves 211, 213, and 214. Further, the flow rate of thecirculation exhaust gas 390 can be similarly adjusted by controlling theopenings of the flow rate regulating valves 213 and 214.

Coal which is fuel is pulverized to pulverized coal by the coal mill130, passes through the primary system pipe 216 together with a part ofthe circulation exhaust gas 390 passing through the flow rate regulatingvalve 215, and then is conveyed to the burner 210. The burner 210 mixesand burns secondary system gas having a high oxygen concentrationflowing through the secondary system pipe 217 and primary system gascomposed of pulverized coal and circulation exhaust gas which flowthrough the primary system pipe 216, thereby generating high-temperaturecombustion gas in the furnace of the boiler 200.

Here, the oxygen concentration in the primary system gas is several %,so a problem arises that pulverized coal hardly ignites in the vicinityof the burner. Therefore, if the oxygen gas 362 is supplied to theprimary system pipe 216, the oxygen concentration of the primary systemgas increases and the ignition property of pulverized coal is improved.However, immediately after oxygen is supplied to the primary system pipe216, a mass of gas having a high oxygen concentration close to pureoxygen is formed inside the primary system pipe 216. If pulverized coalparticles enter the mass of gas, the pulverized coal is oxidized underthe condition of an extremely high oxygen concentration and there is thepossibility that abnormal combustion such as a backfire may occur. Ifabnormal combustion occurs, an extremely high-temperature flame isformed and the burner and the pipe connected to the burner may bemolten. Therefore, to improve the reliability of the oxyfuel combustionboiler plant, a countermeasure for preventing the occurrence of abnormalcombustion is necessary.

Furthermore, the second oxygen supply pipe 397 supplies the oxygen gas362 to the upstream side pipe for supplying circulation exhaust gas tothe coal mill 130. The oxygen concentration of circulation exhaust gasflowing into the coal mill 130 is slightly increased beforehand, thusthe pulverized coal ignition property in the vicinity of the burner canbe improved more.

The constitution of the vicinity of the air separation unit 360 will beexplained below in more detail.

The air separation unit 360 separates the air 363 a to high purityoxygen and a component mainly composed of nitrogen and the separatedcomponent mainly composed of nitrogen is discharged into the air fromthe chimney 370 as the nitrogen gas 361. The oxygen concentration of thehigh purity oxygen is a volume concentration of about 97%. The remaininggas component of 3% is mostly nitrogen. If the nitrogen concentration isexcessively high, the carbon dioxide capture efficiency after combustionis lowered. If the purity of oxygen is made excessively high, the costof the plant is increased. Generally, high purity oxygen is generated ina form of the liquefied oxygen 131. The liquefied oxygen 131 is storedonce in the oxygen buffer tank 132. By doing this, the start of theplant and the switching from the air-fuel combustion to the oxyfuelcombustion can be executed easily. The oxygen buffer tank 132 suppliesthe oxygen stored once to the carburetor 133 and the carburetor 133vaporizes the liquefied oxygen 131 to the oxygen gas 362 and thensupplies it to the boiler 200. A part of the nitrogen gas 361 flowsthrough the nitrogen supply pipe 137. The flow rate regulating valve 134controls the flow rate of the nitrogen gas 361 and then the nitrogen gas361 joins the oxygen gas 362. By doing this, the oxygen purity in theoxygen gas 362 is lowered (the oxygen concentration is lowered). If thisoperation is executed, in the burner inside the boiler, abnormalcombustion such as a backfire hardly occurs.

Further, the operation of supplying the nitrogen gas 361 to the firstoxygen supply pipe 219 is executed only when changing the operationstate of the plant such as switching from the air-fuel combustion to theoxyfuel combustion. In the other cases, the nitrogen gas 361 is notsupplied to the first oxygen supply pipe 219. If nitrogen is alwayssupplied, the nitrogen concentration in the oxygen gas 362 is increasedexcessively and the carbon dioxide capture efficiency after combustionis lowered. Further, to stop the supply of nitrogen, the stop valve 135a is installed on the nitrogen supply pipe 137. The device forcontrolling the flow rate cannot completely stop the supply of gas.Therefore, a stop valve 135 for taking charge of only opening andclosing the flow path is necessary.

In FIG. 1, the vaporized nitrogen joins the vaporized oxygen, thoughliquefied nitrogen may be joined with liquefied oxygen. In this case,the liquefied nitrogen is joined with the liquefied oxygen on thedownstream side of the oxygen buffer tank 132. If the liquefied nitrogenis joined to the liquefied oxygen in the oxygen buffer tank 132 or onthe upstream side thereof, the purity of oxygen is always lowered, sothe carbon dioxide capture efficiency after combustion is lowered.

The aforementioned operation of supplying the nitrogen gas 361 to thefirst oxygen supply pipe 219, when changing the operation state of theplant, is executed to lower the oxygen concentration in the oxygen gas362. Further, to lower the oxygen concentration in the oxygen gas 362,another method may be used. In FIG. 1, the method for supplying air tothe first oxygen supply pipe 219 is also shown. The gas supply unit 136leads air to the air supply pipe 138 and the flow rate regulating valve134 controls the air flow rate and supplies air to the first oxygensupply pipe 219. After completely shifting to the oxyfuel combustion,the air supply from the air supply pipe 138 must be stopped. For thatpurpose, the stop valve 135 is installed on the air supply pipe 138.

As mentioned above, to the oxygen supply pipe 219 on the downstream sideof the oxygen buffer tank 132 and on the upstream side of the junctionof the primary system pipe 216 to the oxygen supply pipe 219, thenitrogen supply pipe for supplying a part of nitrogen 137 generated fromthe air separation unit 360 or the air supply pipe for supplying air 138from the outside is connected, thus an operation of lowering the oxygenconcentration of oxygen gas flowing through the oxygen supply pipe 219can be performed. Therefore, an oxyfuel combustion boiler plant thatprevents abnormal combustion from occurring in the burner can beprovided.

Next, the operation method of the oxyfuel combustion boiler plant willbe explained.

The possibility of an occurrence of abnormal combustion such as abackfire is strongly related to the flame propagation velocity. FIG. 2shows the experimental data indicating the relation between the oxygenconcentration in the oxidizing gas and the flame propagation velocity ofpulverized coal under various combustion conditions. The oxidizing gasis gas to be supplied to the primary system pipe and secondary systempipe of the burner. The flame propagation velocity strongly depends uponthe oxygen concentration in the oxidizing gas. Further, the flamepropagation velocity varies with various conditions in addition to theoxygen concentration. For example, the flame propagation velocity varieswith the factors of the atmospheric temperature, specific heat of theoxidizing gas, property of coal, and particle diameter and concentrationof pulverized coal.

Also, if the flame propagation velocity is extremely high, abnormalcombustion such as a backfire occurs easily. The maximum flamepropagation velocity depends upon the design conditions of the plant.Therefore, so as to control the flame propagation velocity to the designconditions or lower, the combustion conditions must be adjusted.However, if the flame propagation velocity is extremely low, the flamedisappears. Therefore, the flame propagation velocity has an optimumvalue.

The burner at the time of oxyfuel combustion can change the oxygenconcentration. Therefore, a method for adjusting to an optimum flamepropagation velocity is to adjust the oxygen concentration. At the timeof the stationary operation of oxyfuel combustion, the atmospherictemperature, specific heat of the oxidizing gas, property of coal, andparticle diameter and concentration of pulverized coal are decided to acertain extent, so an optimum oxygen concentration is easily decided.Under the control of the oxygen supply rate, the combustion state iseasily controlled.

However, when the operation condition is changed such as switching fromthe air-fuel combustion to the oxyfuel combustion, the atmospherictemperature, specific heat of the oxidizing gas, property of coal, andparticle diameter and concentration of pulverized coal are changedvariously. In this case, it is difficult to decide an optimum oxygenconcentration condition. And, the abnormal combustion phenomenon occursif there is a time zone that the flame propagation velocity exceeds itsupper limit even in a moment and burnout of the combustor may be caused.Therefore, a countermeasure for removing the possibility that the flamepropagation velocity may exceed the upper limit value is necessary.

The countermeasure is to reduce the oxygen concentration under anyatmospheric condition. The countermeasure is effective in the reductionin the flame propagation velocity. For example, from the beginning ofswitching the operation state from the air-fuel combustion to theoxyfuel combustion, the oxygen concentration of the oxygen gas flowingthrough the first oxygen supply pipe 219 is reduced. If the oxygenseparation unit 360 supplies oxygen gas with an oxygen purity of 97%,there is a case that pulverized coal is burnt under the atmosphericcondition of oxygen 97% and the flame propagation velocity at this timeis very high. On the other hand, when the oxygen concentration of oxygengas flowing through the first oxygen supply pipe 219 is reduced to, forexample, 70 to 80%, there are no possibilities that pulverized coal mayburn. From the results shown in FIG. 2, if the oxygen concentration ofthe oxidizing gas is reduced from 97% of oxygen to about 70 to 80% ofoxygen, the flame propagation velocity is reduced to close to ½.Therefore, even by a small reduction in the oxygen concentration, thesuppression efficiency of abnormal combustion is large.

Next, FIG. 3 shows the situation of changes of the fuel supply rate, airsupply rate, oxygen supply rate, and exhaust gas circulation rate untilthe plant starts in the air-fuel combustion, then is switched to theoxyfuel combustion, and reaches the stationary operation state of theoxyfuel combustion.

At the start time, air is supplied from the air pipe for air-fuelcombustion 363 b. At the point in time when the air flow rate reaches apredetermined value, fuel is supplied and ignited. Thereafter, the airsupply rate and fuel supply rate are increased gradually and the load isincreased. At the point in time when the load of switching from theair-fuel combustion to the oxyfuel combustion is obtained, the airsupply rate from the air pipe for air-fuel combustion 363 b is reducedgradually. At the same time, the recirculation of exhaust gas is startedand the exhaust gas supply rate to be recirculated is increasedgradually. In correspondence with the start of the exhaust gasrecirculation, the oxygen supply is started. In correspondence with theincrease in the exhaust gas circulation rate and the reduction in theair supply rate, the oxygen supply rate is increased. At the point intime when the air supply is stopped, the oxyfuel combustion is switchedto. At this point in time, the carbon dioxide capture operation isstarted. Thereafter, the exhaust gas circulation rate, oxygen supplyrate, and fuel supply rate are increased and the necessary conditionsare realized.

The air supply rate, fuel supply rate, exhaust gas circulation rate, andoxygen supply rate at the time of operation are decided beforehand as anoperation plan, though the actual supply rates are slightly shifted fromthe plan. The shifts are caused by errors in the measuring instrumentsfor measuring the supply rates of gas and fuel and the controller. Whenthere are large shifts between the actual supply rates and the plannedvalues, abnormal combustion occurs easily. And, the shifts are causedwhen the operation state is changed such as switching from the air-fuelcombustion to the oxyfuel combustion. For example, when switching fromthe air-fuel combustion to the oxyfuel combustion, the oxygen supply isstarted. To supply oxygen as planned and accurately, the flow ratecontroller must be operated accurately, though the controller always hasan error. From the nature of the controller, the controller easilycauses an error when the flow rate is low. Therefore, when the oxygensupply rate immediately after start of the oxygen supply is low, anerror is easily caused. Further, it is desirable that oxygen is suppliedas slowly as possible, though immediately after supply start, a fixedquantity is easily supplied instantaneously. Further, the flow ratecontroller is always subject to a response delay. Therefore, whenchanging the oxygen supply rate, it is difficult to always adjust thesupply rate accurately. Therefore, even when the operation state ischanged and the oxygen supply rate is shifted from the plan to a certainextent, a plan for the controller to prevent abnormal combustion fromoccurring is necessary.

FIG. 4 shows an example (a comparison example), when the plant isoperated according to the plan shown in FIG. 3, indicating the situationof changes of the oxygen concentration in the oxidizing gas. Thiscomparison example shows the case that from the nitrogen supply pipe 137or the air supply pipe 138 connected to the primary system pipe 216,nitrogen or air is not supplied. Here, with respect to the oxygenconcentration in the oxidizing gas, the average concentration and localand instantaneous concentration must be considered. If the averageoxygen concentration is extremely low, non-ignition may be caused. Acurve 603 shown in FIG. 4 shows an average oxygen concentration. If themain component of the oxidizing gas is changed from nitrogen to carbondioxide, at the same oxygen concentration, the flame propagationvelocity is decreased. Therefore, when it is switched to the oxyfuelcombustion, it is desirable to increase the average oxygen concentrationto a certain extent.

If the local and instantaneous oxygen concentration is extremely high,abnormal combustion such as a backfire is caused. When oxygen issupplied to the primary system pipe 216 through which a mixture ofcombustion exhaust gas and pulverized coal flows, concentrationirregularities are caused in the inner space of the primary system pipe216 and a region of a locally high oxygen concentration and a region ofa locally low oxygen concentration are formed. A curve 604, inconsideration of the concentration irregularities, shows the situationof changes of the oxygen concentration in the region of a locallyhighest oxygen concentration. Inside the primary system pipe 216, themixing property of oxygen gas and combustion exhaust gas varies with theconditions such as the gas flow rate. Therefore, when changing the gasflow rate, it is difficult to always keep a good mixing property andwhen changing the flow rate, there is a case that a local oxygenconcentration is increased temporarily.

A curve 606 shows the upper limit value of the oxygen concentrationallowable to prevent abnormal combustion. The oxygen concentration ofthe oxidizing gas must always be lower than the upper limit value of theoxygen concentration. The oxygen concentration of the curve 604 is lowerthan the upper limit value. However, when switching from the air-fuelcombustion to the oxyfuel combustion, the allowance up to the upperlimit value is reduced. And, when there is a change with time in the gasflow rate, the oxygen concentration may be increased moreinstantaneously. A curve 605 shows an instantaneous highest oxygenconcentration in consideration of the change of the gas flow rate. Asshown in FIG. 4, when the instantaneous highest oxygen concentration 605exceeds the allowable value 606, abnormal combustion occurs easily. Whenthe comparison example is used, there are possibilities that theinstantaneous highest oxygen concentration may reach about 97%. Theoccurrence condition of abnormal combustion is that in addition to ahigh oxygen concentration, a condition that a large volume of pulverizedcoal enters the region of high oxygen concentration is complete. In thisembodiment, the advice for preventing the oxygen concentration fromexceeding the allowable value is made.

FIG. 5, when the plant is operated by applying this embodiment accordingto the plan shown in FIG. 3, shows the situation of changes of theoxygen concentration. If nitrogen gas or air is mixed beforehand in theoxygen gas 362 and the oxygen concentration in the oxygen gas 362 isreduced, the instantaneous highest oxygen concentration can be reduced.For example, if the oxygen concentration in the oxygen gas 362 isreduced by 20%, it can be expected that the instantaneous highest oxygenconcentration is also reduced by 20%. Under the condition that thecontrol for the gas flow rate is most difficult, the oxygenconcentration in the oxygen gas 362 may be reduced to the allowablevalue or smaller. By this operation, the possibility that theinstantaneous highest oxygen concentration may exceed the allowablevalue is eliminated.

FIG. 6 shows a change with time of the oxygen supply rate. It is anexample showing shifts between the planned value and the actual supplyrate. Immediately after start of the oxygen supply, the planned valueand actual flow rate are easily shifted from each other. Inversely toFIG. 6, the actual supply rate may be lower than the planned value.However, it is difficult to artificially control whether to make thesupply rate higher or lower than the planned value. As shown in FIG. 6,when the actual supply rate is higher than the planned value, abnormalcombustion occurs easily. In the reverse case, non-ignition occurseasily. Further, if the oxygen concentration of the oxidizing gas ismeasured and the oxygen supply rate is corrected on the basis of themeasurement results, the error in the oxygen supply rate can be reduced.However, the measuring instrument is accompanied with a response delay.Therefore, before a given period of time elapses after the oxygen supplyis started, the control method using this measuring instrument cannot beused. If a certain period of time elapses and the oxygen supply rate isincreased, the error of the oxygen supply rate is reduced and thecontrol method using the measuring instrument can be used.

FIG. 7, when the plant is operated as shown in FIG. 6, shows thesituation of changes of the instantaneous maximum value of the oxygenconcentration. At the initial stage before a given period of timeelapses after the oxygen supply is started, the oxygen concentrationincreases easily. Further, at this initial stage, the concentration isdifficult to confirm by measurement.

FIG. 8 shows the situation of changes of the supply rate of the oxygengas 362 and nitrogen gas (or air) when this embodiment is applied. Thenitrogen gas or air is supplied slightly earlier than the oxygen supply.Therefore, at the point in time when the oxygen supply is started, thepossibility that high purity oxygen may make contact with a mixture ofpulverized coal and combustion exhaust gas which flow through theprimary system pipe 216 is eliminated. Immediately after start of theoxygen supply, an error is caused easily in the oxygen supply rate, sothe supply rate of nitrogen or air is increased. By doing this, theoxygen concentration of the oxygen gas 362 is reduced, so even if alarge supply rate change or large concentration irregularities arecaused, the instantaneous maximum value of the oxygen concentration ishardly increased. According to the increase in the oxygen supply rate,the supply rate of nitrogen or air is reduced. If the supply rate ofnitrogen or air is reduced, the errors in the nitrogen and oxygen supplyrates are increased. However, at this point in time, the error in theoxygen supply rate is reduced, so the instantaneous maximum value of theoxygen concentration is hardly increased. Further, on the basis of themeasured value of the oxygen concentration, the supply rate can becorrected. At the point in time when it is completely switched to theoxyfuel combustion, the supply of nitrogen or air is stopped. By doingthis, at the time of oxyfuel combustion, high purity oxygen is supplied,so the carbon dioxide capture efficiency in exhaust gas is not reduced.

In the operation state shown in FIG. 8, the situation of changes of theinstantaneous maximum value of oxygen concentration is shown in FIG. 9.At the initial stage of start of the oxygen supply, the oxygenconcentration is reduced. At the initial stage, in the instantaneousmaximum value of oxygen concentration, there exists a principal maximumconcentration. Therefore, at the initial stage that the adjustment ofthe supply rate is difficult, the oxygen concentration can be preventedfrom increasing.

Here, “the principal maximum value of oxygen concentration” will beexplained. For example, when mixing circulation exhaust gas of an oxygenconcentration of 10% with oxygen gas of an oxygen concentration of 70%,the oxygen concentration of the mixed gas becomes smaller than 70%. Atthis time, “an oxygen concentration of 70%” is a principal maximumvalue. As mentioned above, in this embodiment, a principal maximum valueis provided in the oxygen concentration, thus even if the operationcondition is changed, abnormal combustion can be prevented.

As mentioned above, supplying air from the air pipe for air-fuelcombustion 363 b to the exhaust gas recirculation system pipe 390 in anoperation state of air-fuel combustion,

stopping the air supply from the air pipe for air-fuel combustion 363 band supplying and burning the oxygen and the combustion exhaust gas tothe boiler 200 when shifting from the operation state of air-fuelcombustion to the operation state of oxyfuel combustion, and

supplying nitrogen or air to the oxygen supply pipe 219 from thenitrogen supply pipe 137 or the air supply pipe 138 which is connectedto the oxygen supply pipe 219, thus an operation method of an oxyfuelcombustion boiler plant to prevent abnormal combustion from occurring inthe burner can be provided.

Further, as a method for preventing abnormal combustion, when startingthe oxygen supply, a method for making the particle diameter ofpulverized coal larger for supplying may be considered. As shown in FIG.2, if the particle diameter is made larger, the flame propagationvelocity is decreased. Further, if the particle diameter is made larger,the maximum oxygen concentration allowable value 606 shown in FIG. 5 isincreased, so the allowance up to the allowable value is increased andabnormal combustion can be prevented. To make the particle diameterlarger, the operation state of the coal mill 130 shown in FIG. 1 may bechanged. Generally, the coal mill 130 is equipped with a classifier inthe vicinity of the exit thereof. The classifier captures particleslarger than a certain fixed diameter and returns them to the coal millto repulverize. Therefore, to make the particle diameter larger forsupplying, a method for making the separation particle diameter of theclassifier larger may be considered.

Embodiment 2

FIG. 10 shows a modification of the supply method of the oxygen gas 362.The difference from FIG. 1 is that the oxygen gas 362 is supplied to theprimary system pipe 216 in two stages instead of one. By doing this, theoccurrence of abnormal combustion can be suppressed furthermore.

An example of the mixing state of gas and pulverized coal when theoxygen gas 362 is supplied to a flow 31 of the primary system gas isshown in FIG. 11. FIG. 11 shows the mixing state when the oxygen gas 362is supplied at one time.

The oxygen gas 362 is injected from the oxygen supply nozzle 52 towardthe flow 31 of the primary system gas. On the boundary between theoxygen gas 362 and the flow 31 of the primary system gas, a mixingregion 32 is formed. However, the injected gas is not all mixedinstantaneously. Therefore, inside the mixing region 32, a mass of gasof a high oxygen concentration 33 is formed temporarily. The flow 31 ofthe primary system gas is accompanied with pulverized coal particles 34.The pulverized coal particles 34 will not move in completecorrespondence with the flow 31 of the primary system gas. A part of thepulverized coal particles 34 is shifted from the flow 31 of the primarysystem gas and moves independently. As a result, for example, via alocus 36 of pulverized coal particles as shown in FIG. 11, thepulverized coal particles 34 may enter the mass of gas of a high oxygenconcentration 33. The pulverized coal particles 35 entered in the massof gas of a high oxygen concentration burn easily. If the pulverizedcoal particles 35 entered in the mass of gas of a high oxygenconcentration are burnt and raised in temperature, the surrounding gasand pulverized coal are heated. And, the fire spreads easily topulverized coal existing in the mixing region 32 or the flow 31 of theprimary system gas.

To prevent the spreading of the fire, there are two validcountermeasures. The first one is to reduce the oxygen concentration inthe mass of gas of a high oxygen concentration 33 and make thepulverized coal particles 35 entering the mass of gas of a high oxygenconcentration hard to burn. This is the countermeasure indicated inEmbodiment 1. The second one, even if the pulverized coal particles 35entering the mass of gas of a high oxygen concentration are burnt, is toimmediately lower the combustion temperature, thereby making thesurrounding gas and pulverized coal difficult to heat. To make thesurrounding gas and pulverized coal difficult to heat, the volume of themass of gas of a high oxygen concentration 33 is made as small aspossible and the quantity of pulverized coal particles entering the massof gas 33 are reduced. For example, even if pulverized coal enters amass of gas of pure oxygen and burns, if the number of enteringparticles is one, there are few possibilities of spreading of the fireto the surroundings. If the volume of the mass of gas of a high oxygenconcentration 33 is small like this, the number of particles enteringthe mass of gas 33 is reduced and even if the pulverized coal is burnt,the amount of heat due to the combustion can be reduced.

When the volume of the mass of gas 33 is small, the generated combustionheat is soon lost to the surroundings in the direction of an arrow 91,so the temperature does not raise so much. In this case, even ifpulverized coal is burnt once, the fire hardly spreads to thesurroundings. On the other hand, when the volume of the mass of gas 33is large, the fire easily spreads to the surroundings. The combustionheat of the pulverized coal entering the mass of gas 33 is high, so theheat is hardly lost to the surroundings and the temperature riseseasily. As a result, the surrounding gas and pulverized coal are easilyoverheated and the fire spreads easily.

A method for making the volume of the mass of gas of a high oxygenconcentration 33 smaller is preferably a method for dividing andinstalling the exit of the first oxygen supply pipe 219 in the gas flowdirection of the primary system pipe 216, thereby dividing and supplyingoxygen stepwise in the flow direction of the primary system gas. FIG. 12shows an example of the constitution of supplying the oxygen gas 362stepwise. The flow rate of the oxygen gas 362 supplied from the oxygensupply nozzle 52 (one) is reduced, thus the volume of the mass of gas ofa high oxygen concentration 33 is made smaller. Since the volume of themass of gas 33 is made smaller, even if pulverized coal in the mass ofgas is burnt, the fire is hardly spread to the surroundings. Further, inFIG. 12, two masses of gas of a high oxygen concentration 33 are formed.Further, the oxygen supply nozzles 52 are installed away from each otherto a certain extent, thus the mutual action of the masses of gas must beprevented. For that purpose, after the oxygen gas 362 supplied on theupstream side is mixed with primary system gas 10 and the mass of gas ofa high oxygen concentration 33 disappears, so as to newly supply theoxygen gas 362 on the downstream side, the exits of the oxygen supplypipes are desirably installed away from each other.

Embodiment 3

FIG. 13 shows the burner structure and an example of the supply methodof the oxygen gas 362. The primary gas 10 is injected from the centralpart of the burner into the boiler furnace. A flame stabilizer 89accelerates the ignition of pulverized coal. Secondary system gas 217 issupplied from the circumference of the primary system gas 10. The burnerdivides the secondary system gas 217 into two flow paths.

The oxygen supply nozzle 52 is installed inside the primary system pipe216 on the upstream side of the burner. The oxygen supply nozzle 52 isdivided into two parts in the flow direction of the primary system gas10. This method has an advantage in that the primary system gas in theboiler furnace or in the primary system gas immediately after oxygen gasis injected, oxygen concentration irregularities are hardly produced. Ifthe oxygen concentration irregularities are small, there is an advantagethat the combustion properties such as the NOx discharge characteristicand lowest load property can be easily predicted.

Embodiment 4

FIG. 14 shows a modification of the burner structure and the supplymethod of the oxygen gas 362. In this constitution, the oxygen supplynozzle on the downstream side is installed in the vicinity of theburner.

FIG. 15 shows another modification of the burner structure and thesupply method of the oxygen gas 362. In this constitution, the oxygensupply nozzle 52 on the downstream side is installed at the burner exit.At the central part of the burner, a starting oil burner 22 is installedand around the oil burner 22, the oxygen supply nozzle 52 is installed.Around the oxygen supply nozzle 52, a primary nozzle 25 is installed andthe primary gas 10 that is a mixture of pulverized coal and combustionexhaust gas is injected into a boiler furnace 1. Oxygen gas 24 issupplied from the inside of the primary gas 10 injected circularly. Fromthe surrounding of the primary nozzle 25, the secondary system gas 217is supplied into the boiler furnace 1. The secondary system gas isbranched to two flow paths via a wind box 2, then is given the swirlcomponent of the flow by a swirl vane 17, and is supplied into theboiler furnace 1. The oxygen supply nozzle 52 supplies the oxygen gas362.

In the constitution shown in FIG. 15, the oxygen supply nozzle isinstalled on the downstream side of the burner, so there is a defectthat in the primary system gas 10 in the burner portion, oxygenconcentration irregularities are easily produced. On the other hand,when abnormal combustion such as a backfire occurs, there is anadvantage that the influence is hardly exerted upon the upstream side inthe primary nozzle 25.

FIG. 16 shows a modification of FIG. 14. A secondary gas lead-in pipe 51is installed in the vicinity of the burner injection portion. FIG. 17 isa drawing when the burner is viewed in the direction A. On the outerperiphery side of a primary nozzle 401, flame stabilizers 400 areinstalled in a comb teeth shape. Around the primary nozzle, a secondarynozzle 402 and a tertiary nozzle 403 are arranged concentrically. Theoxygen supply nozzles 52 are arranged between the flame stabilizers 400.

1. An oxyfuel combustion boiler plant comprising: an air separation unitfor manufacturing oxygen by separating nitrogen from air, a boilerhaving a burner for burning the oxygen supplied from the air separationunit and pulverized coal and a primary system pipe for supplying thepulverized coal to the burner, an exhaust gas recirculation system pipefor supplying combustion exhaust gas discharged from the boiler to theprimary system pipe, and a carbon dioxide capture unit for capturingcarbon dioxide in the exhaust gas discharged from the boiler,characterized in that: the oxyfuel combustion boiler plant is furthercomprising: an oxygen buffer tank disposed on a downstream side of theair separation unit; an oxygen supply pipe for supplying oxygen to theprimary system pipe of the burner from the oxygen buffer tank; and anitrogen supply pipe for supplying a part of nitrogen generated from theair separation unit or an air supply pipe for supplying air from outsidewhich is connected to the oxygen supply pipe on a downstream side of theoxygen buffer tank and on an upstream side of a junction of the primarysystem pipe.
 2. The oxyfuel combustion boiler plant according to claim1, wherein the nitrogen supply pipe is provided with a stop valvecapable of interrupting a flow of nitrogen.
 3. The oxyfuel combustionboiler plant according to claim 1, wherein an exit of the oxygen supplypipe is divided and disposed in a gas flow direction of the primarysystem pipe.
 4. An operation method of an oxyfuel combustion boilerplant comprising: an air separation unit for manufacturing oxygen byseparating nitrogen from air; a boiler having a burner for burning theoxygen supplied from the air separation unit and pulverized coal and aprimary system pipe for supplying the pulverized coal to the burner; anexhaust gas recirculation system pipe for supplying combustion exhaustgas discharged from the boiler to the primary system pipe; an air pipefor air-fuel combustion for supplying external air to the exhaust gasrecirculation system pipe at time of air-fuel combustion; a carbondioxide capture unit for capturing carbon dioxide in the exhaust gasdischarged from the boiler; an oxygen buffer tank disposed on adownstream side of the air separation unit; an oxygen supply pipe forsupplying oxygen to the primary system pipe of the burner from theoxygen buffer tank; and a nitrogen supply pipe for supplying a part ofnitrogen generated from the air separation unit or an air supply pipefor supplying air from outside which is connected to the oxygen supplypipe on a downstream side of the oxygen buffer tank and on an upstreamside of a junction of the primary system pipe; the operation method ofthe oxyfuel combustion boiler plant is comprising the steps of:supplying air from the air pipe for air-fuel combustion to the exhaustgas recirculation system pipe in an operation state of air-fuelcombustion, stopping the air supply from the air pipe for air-fuelcombustion and supplying and burning the oxygen and the combustionexhaust gas to the boiler when shifting from the operation state ofair-fuel combustion to an operation state of oxyfuel combustion, andsupplying nitrogen or air to the oxygen supply pipe from the nitrogensupply pipe or the air supply pipe which is connected to the oxygensupply pipe.
 5. The operation method of an oxyfuel combustion boilerplant according to claim 4, wherein increasing a particle diameter ofthe pulverized coal for supplying, when the shifting from the air-fuelcombustion operation state to the oxyfuel combustion operation state. 6.The operation method of an oxyfuel combustion boiler plant according toclaim 4, wherein supplying the nitrogen or the air from the nitrogensupply pipe or the air supply pipe is started, prior to start of supplyof the oxygen from the oxygen supply pipe to the primary system pipe,when shifting from the air-fuel combustion operation state to theoxyfuel combustion operation state.